As a conventional type of digital camera, there has been disclosed, for instance, in Japanese Patent Laid-Open Publication No. HEI 2-60378 with the title of "Imaging Apparatus" (First Example of Conventional Technology), an imaging apparatus with improved convenience in use realized by controlling a diaphragm, a shutter speed of an imaging unit and the gain in the variable gain amplifier according to a signal level of an output signal from or an input signal to the variable gain amplifying circuit.
Also disclosed in Japanese Patent Laid-Open Publication No. HEI 1-288070 with the title of "Electronic Imaging Apparatus" (Second example of conventional technology) is an apparatus which controls a gain relating to optoelectronic conversion output from an imaging unit dependent on an exposure rate according to a gain control signal given by a gain control unit, obtains a signal indicating a value corresponding to a shortage of the actual exposure rate from a specified exposure rate as a gain control signal, supplies said gain control signal to said control unit, and continues an imaging operation at a continuously imaging speed set at the point of time with a unit for preserving an operation mode with preference in a continuously imaging speed even under conditions or in a region outside the optimal exposure conditions.
In the second example of conventional technology described above, a shuttering second time (a continuously imaging speed in the continuously imaging mode) set by a photographer can be maintained, and in a case where exposure is short in the continuously imaging mode, the shortage in exposure is compensated by gain control for an image signal, whereby it becomes possible to obtain an image with an optimal quantity of light.
However, in the conventional type of digital cameras as described above, a speed of an electronic shutter can be set only by a pulse XSUB unit for sweeping away an electric charge in a CCD sensor, and for this reason as the shutter speed is set to a higher value, displacement of the actual shutter speed from a theoretical value (quantization error) becomes larger. Detailed description is made below for this phenomenon.
FIG. 9 shows configuration of a representative camera based on the conventional technology. In this figure, the digital camera based on the conventional technology comprises a lens 401, a mechanism 402 including an auto-focus or the like, a CCD 403, CDS circuit 404, an A/D converter 406, an IPP 107, a DCT 108, a coder 109, an MCC 110, a RAM (internal memory) 111, a PC card interface 112, a CPU 421 incorporating a ROM table 421a, a display section 122, an operating section 123, a transfer section 124, a motor driver 425, and an SG (signal generating) section 426. Also a dismountable PC card 150 is connected thereto via the PC card interface 112.
The lens unit comprises the lens 401, the mechanism 402 including an auto-focus (AF)/diaphragm/filter section, and a mechanical shutter in the mechanism 402 simultaneously executes exposure of two fields. The CCD (charge coupled device) 403 converts an image inputted thereto via a lens unit to an electric signal (analog image data). The CDS (correlated double sampling) circuit 404 is a circuit for reducing noises in a CCD type of imaging element. The A/D converter 406 converts analog image data inputted via the CDS circuit 404 from the CCD 403 to digital image data. Namely, an output signal from the CCD 403 is converted to a digital signal at an optimal sampling frequency (for instance, an integer number times larger than a subcarrier frequency of an NTSC signal) by the A/D converter 406.
Also the IPP (Image Pre-Processor) 107 which is a digital signal processing section, the DCT (Discrete Cosine Transform) 108, and the coder (Huffman Encoder/Decoder) 109 divide digital image data inputted from the A/D converter 406 to several portions according to a color difference (Cb, Cr) and brightness (Y), and subject the portions to various types of processing for correcting, compressing or expanding the image. The image compressing/expanding section 107 executes such processing as, for instance, orthogonal transformation or Huffman encoding/decoding, each of which is one step of image compression/expansion based on JPEG standard.
Further, the MCC (Memory Card Controller) 110 stores a compressed image once and records the image via the PC card interface 112 into the PC card 150 or reads out the image from the PC card 150.
Herein controls for operations of the electronic shutter are provided upon input of a control signal con 41 from the CPU 421 according to a control signal group c41 supplied to the CCD 403 by the SG (control signal generating) section 426. FIG. 10 is a timing chart for each of a vertical synchronizing signal VD, a horizontal synchronizing signal HD, a CCD electric charge read pulse XSG1, a continuously variable shutter control signal TRIG, and a CCD electric charge sweep-away pulse XSUB, each of which is included in the control signal group c41.
Also FIG. 10 is a view for illustrating controls over a shutter speed according to the continuously variable shutter control signal TRIG. Namely, when the shutter is operated in he ordinary mode, a terminal for the continuously variable shatter control signal TRIG is set to open or a potential of a power supply unit, but in a case where the shutter is operated in the continuously variable mode, a clock pulse is inputted into the terminal for the continuously variable shutter control signal TRIG.
Namely, the shutter speed is decided by removing a pulse for the CCD electric charge sweep-away pulse XSUB within a time frame of a lagging edge of the CCD electric charge read pulse XSG1 as well as of the continuously variable shutter control signal TRIG and stopping the pulse for the CCD electric charge sweep-away pulse XSUB within a time frame between a lagging edge of the continuously variable shutter control signal TRIG and the next CCD electric charge read pulse XSG1. It should be noted that, in a case where the shutter speed is controlled according to the continuously variable shutter control signal TRIG as described above, a preset value for the shutter speed according to a shutter speed control signal described hereinafter must be set to 1/10000 to broaden the control range.
For the reasons as described above, it is understood that speed setting for an electric shutter can be made only by a unit of CCD sensor electric charge sweep-away pulse XSUB.
Next description is made for control over a shutter speed according to a shutter mode select signal and a shutter speed control signal. At first, upon input of a shutter mode select signal, in the NTSC system, the operation mode is set to any of the high-speed shutter mode in which the shutter speed is faster than 1/60 sec, a low-speed shutter mode in which the shutter speed is slower than 1/60 sec, or a no-shutter mode in which a shuttering operation is not executed.
Then in the high-speed shutter mode or in the low-speed shutter mode, a shutter speed is computed according to the shutter speed control signal. Namely, as shown in FIGS. 11A and 11B, in the NTSC system, a load value is read out from the ROM table 421a incorporated in the CPU 421 for each of the high-speed shutter mode (Refer to FIG. 11A) and low-speed shutter mode (Refer to FIG. 11B), and the shutter speed is computed. Namely, if the load value is set to IPP 107, exposure is executed according to the computed shutter speed.
Assuming that LH is a load value, in a case of the high-speed shutter mode, the shutter speed is computed through the following expression: EQU T=(262D-(1FFH-LH).times.63.56+34.78[.mu.s]
The representative shutter speed obtained through this expression is shown in the chart in FIG. 11A. It should be noted that a figure without any attended character and D as an attended character to a figure indicate a decimal number, and "H" as an attended character to an alphanumerical character indicates a hexadecimal number. With this, in a case of the NTSC system, it is understood that a unit of a shutter speed is 63.56 [.mu.s].
Also in a case of the low-speed shutter mode, the shutter speed is computed through the expression of N=2.times.(FFH-LH) [FLD]. Herein 1 [FLD] indicates 1/60 second, and a product of the FLD value by 1/60 second becomes the specified shutter speed.
Further FIG. 12 shows an example of EV diagram. Herein the EV diagram is a view showing a combination of an aperture value AV (Aperture Value) and a time value TV (Time Value) for achieving a desired exposure value EV (Exposure Value), and there is a relation of EV=AV+TV among the exposure value EV, aperture value AV, and time value TV in the exposure adjustment. A light value Lv (Light Value) is a value obtained by measuring intensity of light, and there is a relation of EV=Lv under appropriate exposure. It should be noted that 1/T is equal to TV-th power of 2, and a square of FNo. is equal to an AV-th power of 2.
In the example of EV diagram shown in FIG. 12, the F value can be set only to either F2 or F8, and for switching it, F2 is selected in a range up to Lv12, and F8 is selected in a faster beyond Lv12. Further, as only F2 and F8 are available for selecting the F value, a desired exposure rate is obtained by changing the shutter speed by a unit of 1/16 TV.
Next description is made for a quantization error in a shutter speed. Charts shown in FIG. 13, FIG. 14 and FIG. 15 are views for illustrating a change of a value against each TV value obtained by a unit of 1/16 TV. Herein the term of the difference (mS) indicates a value obtained by subtracting a time (mS) for the subsequent value from a value for the current value.
Namely, FIG. 13, FIG. 14, and FIG. 15 show how the CCD sensor electric charge sweep-away pulse XSUB unit (63.56 [.mu.s]) affects the exposure. In the figures, the section with .star-solid. indicates the effect, and for instance, the difference between a shutter speed when the VT value is 9.375 and that when the shutter speed is 9.4375 is 0.0639 [mS]=63.9 [.mu.S]. For this reason, a change rate for the shutter speed required to change the TV value by 1/16 when the TV value is around 9.375 is 63.9 [.mu.S], which is around 1 XSUB, and from this it is understood that the TV value may be changed by 1/16 for a change of 1 XSUB.
This is a quantization error due to the CCD sensor electric charge sweep-away pulse XSUB, and summarizing from FIG. 13, FIG. 14, and FIG. 15, influence of 1 XSUB to exposure is as shown below.
______________________________________ Difference in Round number shutter speed by XSUB unit Round number TV (.DELTA.TV = 1/16) (63.56 .mu.S) for .DELTA.TV/XSUB ______________________________________ 8.375 127.7 2 1/32 9.375 63.9 1 1/16 10.375 31.5 0.5 1/8 11.375 16.0 0.25 1/4 12.375 8.0 0.125 1/2 13.375 4.0 0.063 1 ______________________________________
Thus, it is understood that the higher the shutter speed is, the larger a quantization error is.
As described above, in the conventional technology, an operating speed of the electronic shutter can be made only by the CCD sensor electric charge sweep-away pulse XSUB, so that, as a shutter speed is set to a higher value, displacement of the actual shutter speed from the theoretical value (quantization error) becomes larger.
Further, if a diaphragm (iris) can be set without step, the quantization error can be offset by controlling the diaphragm (iris), but in a case where, for instance, the diaphragm (iris) can be set only step by step (as shown in FIG. 12) due to restrictions in system designing for the apparatus or to requirements for cost reduction in the apparatus, the quantization error becomes an exposure error, and when the shutter speed become higher, it may becomes disadvantageously unignorable.